In the production of semiconductor components, it is necessary to be able to set the electrical conductivity of the semiconductor material as precisely as possible according to the desired purpose of the semiconductor components. Semiconductor material for power electronics especially requires unique material characteristics because of the high blocking capability of semiconductor material and the high currents flowing in the semiconductor material. Semiconductor material with non-uniform electrical properties may render the power devices unstable and potentially dangerous.
The electrical characteristics of a semiconductor material are set by doping it with suitable impurity atoms. For power devices to work at designated power levels and voltage readings it is necessary to dope the semiconductor material with impurities that enable sufficient blocking capability and homogeneous current flow through the bulk of the semiconductor material. One effective and accurate doping technique used for high power semiconductor material is the Neutron Transmutation Doping (NTD) process. The conventional NTD process is implemented by irradiating semiconductor ingot rods in a nuclear reactor with neutrons of suitable energy for a suitable time period.
Silicon is by far the most common semiconductor material used for power semiconductor devices today. As applied to silicon, the conventional NTD process provides effective doping control and removal of non-uniformities in high resistivity silicon crystal. When silicon material is exposed to thermal neutron irradiation, phosphorous dopant atoms are induced within the silicon material thereby changing the resistivity of the silicon material. Specifically, when a neutron collides and merges with a 30Si isotope, an unstable 31Si isotope is formed, which subsequently transmutes to a 31P atom by beta decay, resulting in n type impurity doping in the silicon material.
However, since the neutrons are absorbed in the silicon, the conventional NTD process results in an undesired radial gradient of the resulting n-doping, which results in a lateral variation of the electrical characteristics of the power semiconductor devices manufactured from the silicon ingot rod. In order to minimize this effect of inhomogeneity, the silicon ingot rods can, for example, be rotated in the nuclear reactor, so that the neutron irradiation is performed from all sides of the silicon ingot rod. However, despite such measures substantial lateral variations still occur in silicon ingot rods having a diameter of about for example 300 mm or larger due to the above described absorption effects. Lateral doping uniformity of ingot rods having a diameter of about 300 mm or greater has been a challenge using conventional NTD.
Further, the conventional NTD process is implemented with a nuclear reactor which is undesirable. It is difficult to adapt existing nuclear reactors to accommodate larger diameter semiconductor ingots. For example, ingots having a diameter larger than about 200 mm cannot be irradiated in many existing nuclear reactors.